A backlight device which is used as a light source for a liquid crystal display television or the like generally comprises a diffuser panel which is made of acrylic board or the like and arranged in front of the fluorescent lamp. An optical sheet such as a diffusion sheet, a lens sheet or a polarizing sheet is arranged in front of the diffuser panel, and these are held by a chassis (a backlight rear chassis). In such backlight device, especially a direct backlight device in which an irradiation surface is supported in a vertical direction, the ambient temperature around the diffuser panel rises when the backlight device is lit. Then, the diffuser panel expands. Therefore, enough clearance in a horizontal direction is provided between the backlight rear chassis and the diffuser panel.
In a backlight device described in Japanese Patent Application Laid-open Publication No. 2008-090094, a fluorescence tube which is a light source is arranged inside a chassis which is shallow depth box in shape and supported in a vertical direction. The chassis also has a plurality of collinear projections arranged in alignment in a horizontal direction on the upper part of front surface of the chassis. And a diffuser panel is locked by the plurality of projections. The width of the central locking hole among locking holes formed on the diffuser panel is approximately the same as the width of the projection inserted into the locking hole. Moreover, the width of each locking hole of right and left is larger than the width of the projection inserted in the locking hole.